Amplifying system



Dec. 9, 1947. HAMMOND 2,432,151

' AMPLIFYING SYSTEM Original Filed Jan. 18, 1943 5 Sheets-Sheet 1 I H H82 Eye/0a 5x192 e208 [/2 new for laar'ewsfiammofla Dec. 9, 1947.. 1.. HAMMOND 2,432,151

AMPLIFYING SYSTEM Original Filed Jan. 18, 1943 5 Sheets-Sheet 2 case eza-w c230 1721/? for Dec. 9, 1947. HAMMOND 2,432,151

AMPLIEYING SYSTEM Original Filed Jan. 18, 1943 3 Sheets-Sheet 3 j/ZWM 2 6 laarexasflamwzona Patented Dec. 9, 1947 UNITED STATES PATENT OFFICE d PL i yfifiM Laurens Hammond, Qhicago, ill,

r nal. ap icati n r ne als 194s se i l Ne- 7 7 i e an s estimation Ne em her 2 1. 4 Se ia N .119 5 4 Glaims. (Cl. 179-1-71) My invention relates generally to electron dischar tu e amplifying y m and m re rticularly to such systems having improved gain 9m 11 31 In certain uses of electron discharge tube am:- Plii ing Systems, it is desirable to have means for automatically decreasing the gain, or sensitivity of the amplifying system as the amplitude of the input signal increases, and vice versa. An

ap a a u n w i a m fic o S t m f i de. w th he re ha f the i ut signal i one of the amplifiers is of greater amplitude than the input signal to the other amplifier, a signal will appear only in the output of the amplifier having th higher amplitud input signal.

The requirements of the apparatus shown in my aforesaid application are such that the amplifiers shall have an output of substantially uniform amplitude, even though the amplitude of the input signals change by a factor in the order of 100,000.

It is thus an object of my invention to provide an improved amplifying system having automatic gain control means associated therewith.

A iurther object is to provide a two-channel amplifying system with means for automatically controlling the gain of both channels in response to a change in input signal amplitude to either of the channels.

a further object is to provide an improved amplifying system comprising a plurality of channels, and automatically controlling the gain of all of the channels in response to the change in the amplitude of the input signal to any one of the channels.

A further object is to provide an improved amplifying system having a cascaded automatic gain control means associated therewith.

A further object is to provide an improved cascaded automatic gain control means for an amplifying system employing a plurality of decoupled signal channels.

A further object is to provide an improved amplifying system having two separate signal transmitting channels, each channel of the amplifier comprising a plurality of stages, and each stage being provided with an improved automatic shin e ro i cu Qt e b ect wi l ap ea from he. QllQWiflfi- 2 description, reference being had to the accompanying drawings in which: Figures 1a, 1b and 1c taken together constitute a schematic wiring diagram of the novel amplifying system; and,

Figure 2 is a' diagram illustrating the operation 'of the signal limiting electron discharge device.

Referring to Fig. 1a, the amplifying system is shown as having its input provided by two pairs of phototubes 4 6, 41 and 48, 49, each pair of photot'ubes being connected in series between ground and a source of direct current potential indicated as a terminal +90 v., a filter resistor Rl5land a filter condenser Gil-55 being provided in the common portion of the circuits for these pairs of phototubes. It will be noted that the anodes of phototuloes 45 and 48 are connected to the common conductor II) which is normally at+90 v., while the cathodes of the phototubes 46 and i8are connected to the anodes of phototubes 4-! and 49 by conductors I58 and I59 respectively. The cathodes of the latter tubes are connected to ground.

The ,conductor 158 is connected, through a condenser GU and spurious high frequency filtering series grid resistor Rl fiZ, to the grid [64 of a preamplifier pentocle 55. The condenser .1 i o o nected to g u h u h rid resistor R168. In a similar manner condenser Cit! is coupled to the grid I of a preamplifier peutode I 61. The screen and suppressor grids "Q the p ntode 56 enii 1 1 a connected to their plates I'll! and I'll and to conductors ll and I 5 respectively, so that these tubes will opera as triode a will o de cla s A am l atioh- T e oe hedes .7.2 an "3 of these tubes are respectively connected to ground through selfrbias resistors R114 and R415 respectively.

In the apparatus disclosed in my aforesaid application the pairs of phototubes 45, 41 and 48, 49, respectively scan different portions of a field of Vi w i a sc latory man e h t h nifi ant i u siehe im e s d p t r d 1.54, 165 of the preamplifier pentodes I65, I67 w ll b e era l th o m of a single in soidal wave.

Plate current for the preamplifier pentodes H35 and I6! is provided through load resistors Bl as and EH33 respectively, which are connected to suitable +9!) V. terminals of the power supply system. The preamplifier pentode I66 is coupled to a second stage of preamplification, comprising a pentode I85, through a blocking condenser C|8 6, a highrpass f ltering mesh comprising condensers C156 and. C194, and resistors R188, RIQZ and R200, and through spurious high-frequency filier' h se ie gr d. esis or R 96.

The gun 1 93 of-the tube it! is biased through erie freeist r nan which is e heet w a terminal 1.5 v. of a biasing battery 202, the positive terminal of this battery being connected to ground. The cathode 204 of the pentode I84 is connected to ground, while its screen and suppressor grids are connected to its plate 206, plate current being supplied from a +90 v. terminal.

through a load resistor R208. The pentode thus operates as a triode providing a class A amplification. The output to pentode I84 is coupled to the input circuit of a signal amplitude limiting pentode 210 through a blocking condenser 2 I2 and a voltage divider comprising resistors R2I4 and R2I6. The junction 2I8 of resistors R2I4 and R2? is connected through a series grid resistor 220 of the pentode 2I0. The other terminal of the resistor R2 I 6 is connected to a biasing potential source indicated as a 1.5 v. terminal.

The suppressor grid of pentode 2 ID (which may be of the 6W7G type) is externally connected to the grounded cathode 224, while the screen grid 226 is connected to a +45 v. terminal of the power supply system. Plate current is supplied through a load resistor R228 from a +90 v. terminal.

The pentode 2I0 operates in a manner to reduce the amplification of the low value positive peaks of the wave, and to increase the amplification of the negative half of the Wave, as is diagrammatically illustrated in Fig. 2. In this figure the curve 2a represents the grid voltage-plate current characteristic, for negative grid potentials, of a 6W'7G pentode connected in the manner that the pentode 2I0 is shown in Fig. lb.

Two input waves 2b and 2c are indicated on the grid voltage axis 2d. The resultant output waves of the tube are illustrated as 2e and 21 respectively. From this diagram it will be noted that when the amplitude of the input wave, such as 2b, is not very great, the tube operates substantially to cut off the positive peaks of the input wave to the grid and to amplify linearly the negative peaks. The positive portions of the output wave 26 are reduced in amplitude due to the effect of the series grid resistor R220. When the grid potential is positive with respect to the oathode, the grid input impedance falls to a finite value, small relative to the value of resistor R220, with the result that there is a voltage divider action causing the signal on the grid to be greatly reduced. When the input wave on the grid is of much greater amplitude such as represented by the wave 20, the tube operates not only to cut oil the positive portion of the input wave, but also limits the output amplitude due to the negative portion of the input wave.

Briefly the operation of the non-linear pentode 2I9 is as follows: Below a critical threshold amplitude of input signal the tube operates in such a manner as to difierentiate between negative swings of different amplitudes. Above this critical threshold point the tube is unable to distinguish between negative pulses of difierent amplitudes because every such large amplitude pulse drives the tube to plate current cutofi represented by the line 2g. The particular purpose of this type of operation will appear more fully hereinafter.

The output of the pentode 2I0 is connected to the input of an automatic volume control triode 230 through a blocking condenser C232 which also forms part of a high-class filter mesh including resistors R234 and R236. A grid condenser C238 is connected between the junction Of resistors R234, R236 and the grid 240. The grid 240 is connected to a -1.5 v. bias terminal through a reing of this grid to a negative potential greater than the normal grid bias.

The grid condenser C238 may have a value in the order of .1 mid. with the result that it will take an appreciable time interval after a decidedly positive impulse upon the grid 240 before this grid returns to its normal value bias of 1.5 v.

Assuming that the phototubes 46 to 49 scan an area at the rate of 2 C. P. S., the values of the condenser C238 and resistors R242 and R244 are such that it will take more than .5 second (and in actual practice may be in the order of 5 to 10 seconds) for the grid to return to substantially its normal 1.5 v. potential after a substantial amplitude positive signal has been impressed thereon. The result is that as a close succession of positive impulses is impressed upon the grid 240, appreciable changes in plate current will take place only upon the highest amplitude positive impulse.

The plate 246 of triode 230 is supplied with plate current through a load resistor R248 connected to a +90 v, terminal, and the signal component of the plate current is transmitted through a blocking condenser C250, through a voltage divider network comprising resistors R252 and R254 and a grid resistor R256, to the control grid 25B of a phase inverting triode 260. The cathode 262 of this tube is connected to ground through a biasing resistor R264. Plate current for the triode 250 is supplied through a load resistor R266 from a +90 v. terminal.

The triodes 230 and 260, and associated circuit elements, comprise a single stage of amplification and volume or gain control. The output of the phase inverter triode 260 is transmitted through a band pass filter mesh 210 to the input of an automatic volume control triode 216, and the output of the latter is transmitted through high pass filter and voltage dividing mesh 289 to a phase inverting triode 282.

The output of the triode 282 is coupled to the input of an automatic volume control triode 286 through a band pass filtering mesh 299. The output of the triode 286 is coupled with a voltage divider and high pass filtering mesh 292 to the input of a phase inverting triode 296.

The output of the phase inverting triode 296 is transmitted through a blocking condenser C298 to a conductor I6 which is connected to ground through a coupling resistor R300. The conductor I6 may be connected to any suitable device or apparatus which is to be operated or controlled by the signal initially generated in the phototubes I46, I41.

Triodes 216 and 282, and the circuit elements associated therewith, are in substance identical with triodes 230 and 260 and the circuit elements associated therewith, and thus form a second cascaded stage of amplification and automatic volume control. Triodes 28B and 296 and associated circuit elements are likewise similar to triodes 230 and 260 and their associated circuit elements, and thus constitute the third and final cascaded stage of amplification and automatic volume control.

The output of the amplifier tube I61 (Fig. la) is coupled through conductor I5 to the input of the second stage pre-amplifier pentode I which,

a rears-1 with its associated circuit elements, corresponds to the" pentod'e till above described. Likewise pentode 2H corresponds to pentode 2N? and is coupled tothe tube I85 in the same manner that the't'ube2lll is coupled to' the tube I84. Similarly triodes 23I, 251, 2W, 283; 2'3? and 29? together with their associated circuit elements, correspond respectively to triod'es 2'39, 26:}, 2'16, 282, 28-6 and 2% and their associated circuit elementsrespectively. The output of the triode 291' is transmitted through a blocking condenser C299 to a conductor' H, the latter being connected to ground through a coupling resistor R391.

While" the above described amplifier having elements bearing odd numbered reference characters (lower channel) may be identical with the corresponding parts of the amplifier whose elements bear' even numberedreference characters (upper channel), it will be noted that the bias voltage'fortriodes 239 and 23l is supplied through a common high value resistor REM and individual relatively low value resistors R242 and R243 respectively; As a result, the grid bias on triodes 2'31) and 231' will be substantially the same at all times. From this it will be apparent that as one of" the amplifier triodes is made less sensitive by having impressed thereon a high amplitude signal, the sensitivity of the other triode is correspondingly reduced.

In thesame way automatic gain control triodes 275- and 27'! are supplied with biasing Voltage through a common resistor R214 of high value (in the order of 50 megohms) which has one terminal connected to a l.5 v. terminal, while the grids of triodes 218 and 271 are connected to the other terminalof resistor R274 through relatively low value (in the order of 3 megohms) resistors R212 and R213.

Thus, these corresponding stages of the two amplifiers are likewise retained at. substantially equal sensitivity. Similarly the system, let us assume that the phototubes are utilized to scan a field in which there is a distant source of light of a frequency at which the phototubes 46 to 49 are sensitive. At the initial great distance between the source of light and the phototubes, other sources of light of less intensity which may be present in the scanned field are assumed to be of low intensity compared with the source of the radiation which the system is to detect. As a result the signal output of the pair of phototubes which is scanning the source will comprise a very low amplitude impulse, presumably in the shape of substantially a single wave, which may depart from a sine wave shape, depending upon the shape of the source of radiation and the character of the scanning apparatus used. In addition to the single wave resultant from scanning the particular radiation source of highest intensity, other sources of light radiation or sources by reflection) may likewise produce fluctuations in the output of the phototubes. The signal-s produced as a result of scanning these minor sources will, by definition, be of lower amplitude than the signal produced upon scanning the high intensity radiation source. The signal from the phototubes 46, 4'! will be faithfully transmitted through the preamplifier pentodes and- |'8"4= and will be impressed upon the grid of the limiter pentode 21%. This tube, due mainly toits series grid resistor R220; responds only to the relatively high amplitude negative peaks of the received signal, and thus will tend to transmit the positive peaks of low substantially uniform amplitude as indicated by the curves 2e and 2)- of Fig. 2.

Assuming that the distance between the source of radiation and the scanning apparatusis decreasing, the impulses produced in the phototubes, as the result of scanning minor discontinui'ties' or irregularities in intensity of radiation throughout the field scanned by the phototubes, will become progressively of relatively high amplitude'; The amplifier, however, operates in such mannerthat signals from the phototubes due to such" minor sources" of radiation are ignored. This" is accomplished because of the fact that the triode 286, dueto the previous regular reception of high amplitude signals. is biased so far nega- Since the signal is amplified to the greatest extent in this last stage, represented by the triode '2'8'6', this will be the first automatic volume control triode to be rendered insensitive to any but the higher amplitude positive peaks of the input signal.

As the scanningapparatus and source approach each other more closely, the amplitude of the signals impressed upon the triode 216 will be such as to increase negatively the bias on this triode and make it capable of transmitting only the highest positive peak of its input signals. Similarly, as the scanning apparatus andsource approach each other still more closely, the input signal on the triode 230 becomes of such high amplitude that this triode is also biased negati'vely to an extent such that it is, in effect, cut oil. except for the highest amplitude positive signal of its input. Thus the signals due to other minor sources of. radiation are not transmitted through the amplifier, but are eliminated. from the final output signal due to the Operation of the several stages of automatic gain control.

Because of the use of the limiter tube 2! and the successive stages of automatic volume or gain control whereby the sensitivity of the amplifier as a whole decreases successively as the amplitude of the highest peak signal received increases, the amplifier is capable of supplying a series of significant output signals of nearly constant amplitude between output conductor l6 and ground which may be utilized to operate or control other apparatus.

The phototubes 48 and 49 may be arranged to scanan area adjacent (and possibly to some extent overlapping) that of the area scanned by the phototubes 46 and 41 and thus, if the principal source of light appears in one of these areas, it will condition the amplifier channel associated with the phototubes scanning that area to respond only to radiation from the selected source. However, it is possible that the direction of the scanning apparatus, or the position of the principal source may change so as to cause the source to disappear from the area scanned by the phototubes 46 and 41 and to appear in the field scanned by the phototubes 4'8 and 49. In order that the lower amplifying channel coupled to the phototubes 48 and 49 may be in condition to transmit to its. final output conductor only the signal resultant from scanning the selected source (and not signals resulting from scanning minor sources of radiation), the sensitivity of the lower amplifier channel is at all times maintained the same as that of the upper channel. This is accomplished by virtue of the fact that a change in the grid bias upon either triode 216, 286 or 230 is reflected in a corresponding change in the bias applied to triodes 211, 281 and 23I respectively, since each of these pairs of tubes has essentially a common source of grid bias.

As a result of this arrangement the source of light radiation of the greatest intensity present in either of the fields scanned by the pairs of phototubes will result in the transmission of a signal to the output conductor I6 or H, depending upon whether the principal radiation source is in the field scanned by the phototubes 46, 41 or in the fields scanned by the phototubes 48, 49. Other irregularities in the intensity of radiation in different portions of the field scanned will be eliminated or blocked out so as not to provide any signal to either of the output conductors l6, [1.

From the foregoing it will appear that I have provided an improved amplifying system which comprises a plurality of cascaded stages of automatic gain control. It will be noted that the gain control differs materially from various types of automatic volume controls as used in radio receivers, in that the elements of the gain control circuit for each stage are individual to that stage and that the control is effected by the amplitude of the signal supplied to the stage. The range of overall gain or amplification of the amplifier is geometrically increased as the number of stages is arithmetically increased. The range of amphfication which may be obtained, while producing an output signal 'of relatively uniform amplitude, is thus limited only on the one hand by the fact that the signal supplied to the first stage must be of sufficient amplitude relative to unavoidable thermal and similar noise that the signal may be significantly amplified in the first stage, and on the other hand by the fact that the input signal shall not be of higher voltage than can safely be permitted by the design of the vacuum tube to which the input for the first stage is supplied.

It will further appear that the multi-channel amplifying system disclosed herein provides a simple and novel arrangement whereby all except the highest amplitude signal impulses derived from a plurality of sources may be blocked while permitting the highest amplitude signal to pass through a transmission channel individual to the source from which such signal originated.

While in the system disclosed herein there are but two transmission channels, it will be apparent that the invention may be utilized in systems employing three or more channels, since in the latter case it would be necessary merely to have a common grid resistor (such as the resistor R244) for the corresponding stages of the three or more amplifying channels.

While I have shown and described a particular embodiment of my invention, it willbe apparent to those skilled in the art that numerous modifications and variations may be made without departing from the underlying principles of the invention. I therefore desire, by the following claims, to include within the scope of my invention all such variations and modifications of the construction disclosed by which substantially the results of the invention are obtained through the use of substantially the same or equivalent means.

I claim:

1. A pair of amplifiers each automatically de-;-

creasing its sensitivity as the input signal to either of them increases in amplitude, each comprising means for converting its input signal into a series of impulses, an electron discharge device having a cathode, grid and plate, and means for impressing impulses upon said grid; a high impedance grid bias potential source common to both of said amplifiers, and a resistance and capacitance network connected between said grids and said grid bias potential source, said network including a resistance of such high value between each of said grids and said source that grid rectification will cause the grid bias to increase in a negative direction to a value such that the device will be cut 011' except for the most positive peak portions of the input signal.

2. In a system for amplifying electrical signals from two related sources, the combination of two separate amplifiers for respectively amplifying the signals from said sources, each of said amplifiers comprising a plurality of cascaded stages; each of said stages comprising a phase inverter and a band-pass filtering resistance and capacitance network; and a plurality of automatic gain controlling means, each of said means being coupled to two corresponding stages of the two amplifiers and being substantially equally responsive to the signal amplitudes at the two stages to which it is coupled and operable to control the gain of said two stages.

3. In a system for amplifying electrical signals from a plurality of related sources, the combination of a plurality of separate amplifiers for respectively amplifying signals from said sources, each of said amplifiers comprising a plurality of cascaded amplifying stages, each of said stages comprising a phase inverter and a band-pass filtering resistance and capacitance network; and

a plurality of automatic gain controlling means, each of said means being coupled to all corresponding stages of the amplifiers and substantially equally responsive to the signal amplitudes of all of the corresponding stages, and operable to control the gain of all of said corresponding stages.

4. In a system for amplifying electrical signals from a plurality of related sources, the combination of a plurality of separate electron discharge device amplifiers for respectively amplifying signals from said sources, and a plurality of independent cascaded stages of automatic gain controlling means in each of said amplifiers, the corresponding stages of all of said amplifiers having a common element in their respective gain controlling circuits effective to cause the gain of all corresponding stages to decrease substantially to the same extent as the signals from any of said sources increase in amplitude.

LAURENS HAMMOND.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number Name Date 2,226,994 Schlesinger Dec. 31, 1940 2,318,338 Simon et al May 4, 1943 2,176,663 Browne et a1 Oct. 17, 1939 2,170,835 Simon Aug. 29, 1939 FOREIGN PATENTS Number Country Date 22,396/35 Australia Sept. 28, 1936 

